1. Field of the Invention
This invention relates to mixtures of isocyanate-terminated polyurethanes having good adhesion to metal and polymeric substrates and to the use of such mixtures as hot melt adhesives, coatings and/or sealants.
2. Description of the Related Art
Isocyanate-terminated polyurethane prepolymers (sometimes referred to hereinafter as "polyurethane prepolymers" or "prepolymers") are desirable in a variety of applications. For example, they can be used in reactive hot melt urethane adhesive, coating and/or sealant systems. Hot melt urethane systems are solid at room temperature, melt to a viscous liquid when heated to moderate temperatures (e.g., 82.degree.-121.degree. C.), and are applied in a molten state to an appropriate substrate. They then cool to a solid state to provide an initial bond strength (sometimes referred to as "green strength"), and eventually achieve their ultimate bond strength in a curing reaction with ambient moisture.
An adhesive which can readily bond to a variety of metallic substrates is useful in numerous applications, especially where other techniques of metal attachment, for example, welding, brazing, soldering, mechanical fasteners (e.g., bolts, screws, rivets, clips and the like), adhesive tapes, or solvent-based chemical bonding agents are undesirable or not feasible. An adhesive which can readily bond to polymeric substrates, for example, polystyrene or polymethylmethacrylate are also useful because such substrates previously have not been amenable to adhesive bonding.
U.S. Pat. No. 3,931,077 to Uchigaki et al. discloses a reactive hot melt composition comprising: (a) 20-73 wt. % of a urethane prepolymer prepared from a diol selected from ethylene glycol, propylene glycol, 1,4-butanediol, 1,4-butinediol (sic), polymers thereof and poly(tetramethylene glycol) ether (sic); (b) 2-25 wt. % of a thermoplastic resin of an ethylene-vinyl acetate copolymer, an ethylene-acrylic acid copolymer, an ethylene-acrylate copolymer, an atactic polypropylene or a poly(ethylene terephthalate) linear polymer; and (c) 25-55 wt. % of a tackifier. It is stated that the tackifiers impart high cohesive force at temperatures below 60.degree. C. and high instant adhesive strength. It is also stated that the compositions have a long pot-life, high instant adhesive strength, and good application temperature characteristics. A blend of prepolymers based on an essentially semicrystalline hydroxy-functional material, an essentially amorphous hydroxy-functional material, and poly(tetramethylene ether)glycol is not disclosed.
U.S. Pat. No. 4,352,858 to Stanley discloses a modified polyurethane adhesive composition comprising an isocyanate-terminated prepolymer and a dicarbamate ester which is compatible with the prepolymer and which contains two free isocyanate groups. It is stated that the dicarbamate coreacts with the prepolymers and becomes part of the cured polymer matrix, thereby contributing to the properties of the cured polymer. A blend of prepolymers based on essentially semicrystalline and essentially amorphous hydroxy-functional materials and poly(tetramethylene ether)glycol is not disclosed.
British Patent No. 2,137,638 discloses a urethane hot melt adhesive composition which comprises a moisture-curable polyurethane formed from a diisocyanate, a substantially linear hydroxy polyester, and a monofunctional reactant present in an amount sufficient to react with up to 40 mole percent of the isocyanate groups of the product of the prepolymer reaction. Supposedly this provides a coating composition which is susceptible to moisture but which does not degrade during storage of a coated substrate. That is, the coating is no longer heat softenable. A blend of polyurethane prepolymers based on an essentially semicrystalline hydroxy-functional material, an essentially amorphous hydroxy-functional material, and poly(tetramethylene ether)glycol is not disclosed.
U.S. Pat. No. 4,585,819 to Reischle et al. discloses a polyurethane hot melt composition which comprises an isocyanate prepolymer, a thermoplastic polyurethane or polyester, and/or a low molecular weight synthetic resin selected from the group consisting of ketone resins, hydrogenation products of acetophenone condensation resins, and mixtures thereof. The resin is stated to improve hot tack, wetting capacity, and the binding velocity of the not crosslinked adhesive. A blend of polyurethane prepolymers based on essentially semicrystalline and essentially amorphous hydroxy-functional materials and poly(tetramethylene ether)glycol is not disclosed.
U.S. Pat. No. 4,820,368 to Markevka et al. discloses a reactive hot-melt urethane adhesive composition comprising a polyurethane prepolymer composition which is the reaction product of a polyalkylene polyol, an isocyanate compound, a thermoplastic block copolymer, and an aliphatic, aromatic or mixed aliphatic-aromatic tackifying resin. Bonds formed with the adhesive are said to have good heat stability, initial bond strength and pot stability. A blend of polyurethane prepolymers based on essentially semicrystalline and essentially amorphous hydroxy-functional materials and poly(tetramethylene ether)glycol is not disclosed.
European Patent Application Publication No. 0 340 906 discloses a hot melt polyurethane adhesive composition which comprises a mixture of at least two amorphous polyurethane prepolymers characterized in that each prepolymer provides a different glass transition point to the composition. It is stated that while the separate prepolymers do not give satisfactory properties, a mixture of the two prepolymers does. A blend of polyurethane prepolymers which is based at least in part on poly(tetramethylene ether)glycol is not disclosed.
Several articles by H. F. Huber and H. Muller ("Moisture Curing Hot Melts," 1988 Hot Melt Symposium, Tappi Notes, published by Tappi Press, Atlanta, Ga., "Structural Bonding with Polyester Hot Melt Adhesives," 1990 Hot Melt Symposium, Tappi Notes, published by Tappi Press, and "Shaping Reactive Hot Melts Using LMW Copolyesters," Adhesives Age, November, 1987) disclose the combination of isocyanate-capped amorphous and crystalline polyesters, which combinations are said to provide good adhesion to various specific substrates. Formulations which include an isocyanate-terminated poly(tetramethylene ether)glycol are not disclosed.